Variation in the zero-point energy difference via electrostatic interactions in Co(ii)-Cltpy-based spin-crossover molecular materials†‡

Abstract

Herein, newly synthesized Co(II)-Cltpy-based spin-crossover molecular materials composed of ClO₄⁻, BF₄⁻, PF₆⁻, and CF₃SO₃⁻ are systematically investigated, emphasizing their structure–property interplay. The multi-faceted character of the ligand 4′-chloro-2,2′:6′,2′′-terpyridine (Cltpy) causes the Co(II)-based molecular materials to show electronical behavior crucial to spin crossover involving crystallographic packing and disparity in the zero-point energy difference, ΔE⁰_HL, through electrostatic interactions mediated by their cationic and anionic moieties as well as other dipolar and quadrupolar moieties. All crystalline coordination networks were found to be phase-pure, well-crystallized, and geometrically constrained, subsequently demonstrating fascinating crystal packing along with a gradual, incomplete temperature-dependent magnetic response and negligible elastic interactions among the Co(II)-spin-crossover centers. The ClO₄⁻ and PF₆⁻ analogs showed comparatively strong intra-chain and inter-chain interactions, resulting in substantially strong effective crystal fields experienced by the Co(II)-spin-crossover centers. This indicated large positive values of ΔE⁰_HL and their distribution with considerable stabilization of the low-spin state over the high-spin state at higher temperatures, indicating an identical magnetic response close to the pure low-spin state. Alternatively, the BF₄⁻ analog demonstrated comparatively weak intra-chain interactions without having any inter-chain interactions, resulting in moderately weak effective crystal fields around the Co(II)-spin-crossover centers. This signified negative values of ΔE⁰_HL and their distribution towards stabilization of the high-spin state over the low-spin state, as exhibited by the enhanced magnetic response up to room temperature. In the case of the CF₃SO₃⁻ analog, discrete molecular moieties were observed without any intra- and inter-molecular interactions, which was attributed to its weakest effective crystal-field strength among all the compounds reported herein, essentially signifying moderately large negative values of ΔE⁰_HL and their distribution towards stabilization of the high-spin state over the low-spin state, as indicated by the rapid increase in the magnetic response to the greatest extent. Further, the EPR data of the reported compounds at 8 K showed an excellent fingerprint for the formation of the LS Co(II). The significant variation in the polarity of the individual counter anions played a vital role in the alteration of the crystal packing, along with varying intra-, inter-molecular, and electrostatic interactions. The experimental results were further integrated with the newly developed physically interpreted theoretical model (electrostatic-mechanoelastic model) to highlight the role of electrostatic interactions in combination with the alteration in the crystallographic packing for successfully reproducing the experimental magnetic behavior.